The present invention relates generally to a spindle motor, and more particularly, to a brushless spindle motor used in a dental handtool.
Dental handtools that include a handpiece having a drill and a spindle motor to rotate the drill are generally known. The spindle motor used in a handtool should rotate fast. It is therefore typical to use a brushless spindle motor that includes a rotor having multiple magnetic poles. Multiple armature coils are then provided around the rotor.
Conventional rotor are rotatably supported in the motor casing by a ball bearing. This causes the bearing to wear relatively quickly, thereby reducing the durability of the dental spindle motor that always rotates at a high speed.
To solve this shortcoming, the present inventor employed a pneumatic bearing as a rotor bearing means, and developed a spindle motor which supports the rotor in a non-contacting manner. The first developed spindle motor includes a rotor having a plurality of internal magnetic poles. A cylindrical sleeve rotatably supports the rotor and a plurality of armature coils are provided on the outer surface of the sleeve. Compressed air is then introduced from an external source into a clearance between the rotor and the sleeve, forming a pressurized air film in the clearance to lift the rotor from the sleeve.
When such a tool is in use, the rotor sometimes contacts the inner surface of the sleeve and slides. Further, the armature coils should be placed directly on the outer surface of the sleeve to decrease the diameter of the motor, and the thickness of the sleeve is accordingly limited. The sleeve therefore has to be formed of a material that creates little friction when it contacts the rotor. Additionally, high abrasion resistance, mechanical strength, and electrical insulating characteristics are also required. In this respect, silicon carbide was initially considered the most appropriate material for the sleeve.
However, when a silicon carbide sleeve is used in the spindle motor, the rotational speed of the rotor does not sufficiently reflect the amount of current supplied to the armature coils. Indeed, the smaller the motor is, the more prominent this phenomenon tends to be. It is therefore desirable to solve this problem in order to increase the practical applications for spindle motors with pneumatic bearings.
As a result of the intensive study of the above-described phenomenon, the present inventor has found that a magnetic field formed by the armature coils induced a current in the silicon carbide sleeve which heats the sleeve. A portion of the magnetic energy from the armature coils is therfore wasted as thermal energy.